patch "cdc-acm: prevent infinite loop when parsing CDC headers." added to usb-linus

This is a note to let you know that I've just added the patch titled

cdc-acm: prevent infinite loop when parsing CDC headers.

to my usb git tree which can be found at
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb.git
in the usb-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From: Quentin Casasnovas <[email protected]>
Date: Tue, 14 Apr 2015 11:25:43 +0200
Subject: cdc-acm: prevent infinite loop when parsing CDC headers.

Phil and I found out a problem with commit:

7e860a6e7aa6 ("cdc-acm: add sanity checks")

It added some sanity checks to ignore potential garbage in CDC headers but also introduced a potential infinite loop. This can happen at the first loop iteration (elength = 0 in that case) if the description isn't a DT_CS_INTERFACE or later if 'buffer[0]' is zero.

It should also be noted that the wrong length was being added to 'buffer' in case 'buffer[1]' was not a DT_CS_INTERFACE descriptor, since elength was assigned after that check in the loop.

A specially crafted USB device could be used to trigger this infinite loop.

---
 drivers/usb/class/cdc-acm.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

index 3e15add665e2..5c8f58114677 100644
@@ -1142,11 +1142,16 @@ static int acm_probe(struct usb_interface *intf,
 	}
 
 	while (buflen > 0) {
+		elength = buffer[0];
+		if (!elength) {
+			dev_err(&intf->dev, "skipping garbage byte\n");
+			elength = 1;
+			goto next_desc;
+		}
 		if (buffer[1] != USB_DT_CS_INTERFACE) {
 			dev_err(&intf->dev, "skipping garbage\n");
 			goto next_desc;
 		}
-		elength = buffer[0];
 
 		switch (buffer[2]) {
 		case USB_CDC_UNION_TYPE: /* we've found it */

2.3.6